The present invention relates to a hot strip coiling mandrel capable of preventing top marks (indentations or impressions) caused at the leading end of the coil during coiling a relatively thick hot strip of, for example, steel.
In hot strip mill operation, the production of relatively thick strips such, for example, as steel plates having a thickness of 6 mm or more has recently been increasing. For coiling such a relatively thick strip, there is generally used a mandrel having as a coiling shaft an assembly of a core shaft and a plurality of drum segments circumferentially divided into, for example, four equal parts. While a mandrel of this type is capable of expanding and collapsing radially by adjusting the core shaft or the drum segments, it has a serious disadvantage in coiling a relatively thick strip that top marks (indentations or impressions) are caused in the portions of the strip overlapping the leading coil end fastened to the mandrel around three to five turns because of the difference in the thickness of the coiled strip since the cross section of the coil is maintained always nearly in a right circle, thereby making it necessary to scrap the leading end portion of the strip having a length of 5 m to 10 m.
In order to overcome this disadvantage it has been proposed to provide pressure rolls such as wrapper rolls or buffer means. However, this has not yet achieved satisfactory results.
In coiling a relatively thin strip, on the other hand, since there is no possibility of the occurrence of top marks, no buffer means is required. Rather, provision of a buffer means will cause slippage between the mandrel and the strip. Accordingly, in coiling such relatively thin strip using the mandrel provided with a buffer means, there must also be provided means for keeping the buffers from carrying out their buffer function.